The catalyst, which is used for hydrogenation refining, is typically prepared by allowing a porous alumina carrier to carry an active metal belonging to the group VIa and the group VIII of the periodic table. It is necessary for such an alumina carrier to have a large surface area in order to uniformly disperse the active metal. For this purpose, it is desirable to possess a large volume of the pore called "mesopore" having a diameter in a range from several tens to several hundreds of angstroms. On the other hand, it is also required to possess a specified size of pore diameter in order that a reactant arrives at an active surface at the inside of the catalyst pore, and a reaction product is released therefrom. In order to satisfy the request as described above, it is important that not only the pore diameter but also the pore diameter distribution is adjusted in conformity with the reactant.
For example, when the atmospheric residual oil or the vacuum gas oil is decomposed and processed with a catalyst, then the oil having a large molecular weight is decomposed during the decomposition reaction, and the pore of the catalyst (catalyst carrier) is closed. As a result, the activity of the catalyst is lowered. Therefore, when the atmospheric residual oil or the vacuum gas oil is processed as described above, it is necessary to use a catalyst carrier which has a sharp pore diameter distribution having a peak within a specified pore diameter range corresponding to the molecular weight of the oil to be processed and which has a large total pore volume concerning pores belonging to the specified pore diameter range.
It has been hitherto revealed that when the catalyst for hydrogenation refining is produced, the catalytic activity can be maintained for a long period of time by selecting the pore diameter distribution of the catalyst carrier while considering, for example, the type of the raw material oil, the reaction condition, and the catalyst particle diameter. Those known as the method for adjusting the pore diameter distribution include, for example, a method in which an alumina raw material power is added with an additive such as a fine powder of a water-insoluble organic polymer, a water-soluble organic solvent such as alcohol, carboxylic acid, and ketone, and a surface active agent such as higher alcohol and higher alkylamine. After the fine powdery fire-resistant inorganic oxide or hydroxide is formed, the additive is removed by means of the drying and calcining treatment. Another method has been also suggested, in which the pore diameter distribution of the catalyst carrier is adjusted by treating the formed, dried, or calcined carrier or catalyst, for example, with hot water, acid, aqueous solution of metal salt, or steam.
However, even when the method as described above is used, it is difficult to obtain the catalyst carrier having the pore diameter distribution which is most suitable, for example, for the application or use such as the treatment for the atmospheric residual oil or the vacuum gas oil. Further, there has been a problem that the catalytic activity of the obtained catalyst cannot be maintained for a long period of time.
The following method is adopted when the alumina carrier is produced. That is, a peptizing agent such as acid or alkali is added to a pseudo-boehmite powder followed by mixing and kneading to form a pellet which is then dried and calcined. A technique concerning this method is known, in which the calcination condition such as temperature is changed when the formed pseudo-boehmite powder is calcined, in order to regulate the pore diameter of the alumina carrier. However, the final pore diameter distribution of the carrier is substantially determined by the pore diameter distributions of the raw material pseudo-boehmite powder and the formed product after the mixing and kneading. Therefore, it is difficult to perform the control so that the pore volume of pores belonging to those having a specified pore diameter range is increased merely by changing the calcination temperature and the calcination atmosphere.
It is assumed that the step other than the calcination step, which makes it possible to regulate the pore diameter distribution, is the mixing and kneading step for the pseudo-boehmite powder. In order to obtain the pore diameter distribution having a large pore volume concerning a specified pore diameter by means of the mixing and kneading process, important factors are the peptizing property of the powder and the size of the primary and secondary particles of the pseudo-boehmite powder. Some pseudo-boehmite powders are inferior in peptizing property, and they cannot be peptized with acid or alkali, resulting in an alumina carrier having a broad pore diameter distribution. Therefore, in order to obtain an alumina carrier having a sharp pore diameter distribution, it is necessary to use a pseudo-boehmite powder having excellent peptizing property.
Japanese Patent Publication No. 6-8174 discloses a method for producing a pseudo-boehmite, in which a solution of alkali aluminate is added to an aqueous solution of mineral acid salt of aluminum in the presence of hydroxycarboxylic acid to obtain a slurry to which an aqueous solution of mineral acid salt of aluminum and a solution of alkali aluminate are then simultaneously added so that precipitate of aluminum hydroxide is produced in two stages in a separated manner. It is reported in this patent document that an obtained pseudo-boehmite powder is excellent in peptizing property, and an alumina carrier having a sharp pore diameter distribution can be produced by using the obtained pseudo-boehmite powder as a material for producing the catalyst carrier. However, as disclosed in Japanese Patent Application Laid-Open No. 8-10627, a carrier having a sharp pore diameter distribution, which is produced from a pseudo-boehmite powder having an excellent peptizing property, is inferior in water stability. As a result, a new problem arises in that the strength is lowered during the step of impregnation with a catalyst metal solution.
An object of the present invention is to provide a pseudo-boehmite powder to be used to produce an alumina catalyst carrier which has a sharp pore diameter distribution so as to increase the pore volume concerning a pore diameter of 60 to 120 .ANG., especially within a specified pore diameter range within this range, and which undergoes less decrease in strength when the carrier is impregnated with a catalyst metal salt solution. Another object of the present invention is to provide a catalyst carrier for hydrogenation refining, based on the use of the pseudo-boehmite powder as a raw material powder, the catalyst carrier being preferably used for the hydrogenation refining process such as the desulfurization process and/or the denitrification process for a petroleum fraction within a specified boiling point range.